1. Field of the Invention
The present invention generally relates to gettering technique applied to a semiconductor wafer having an SOI (Silicon On Insulator) structure (hereinafter referred to as SOI wafer). Gettering is a process of removing from a device region heavy metal impurities such as Fe, Cu, Cr, Ni and Pt that have an adverse effect on the characteristics of a semiconductor device.
2. Description of the Background Art
Referring to FIG. 58, a bulk silicon wafer is usually formed by depositing on the back of a wafer formed of a silicon layer 1 a polycrystalline silicon layer 9 which serves as a gettering layer.
SOI wafer has been attracting attention as a silicon substrate of the next generation since SOI is effective in preventing the short channel effect and CMOS (Complementary Metal-Oxide Semiconductor) latchup that are found problematic as the semiconductor device is further decreased in size, and it achieves a high speed operation even in a low voltage region. Referring to FIG. 59, SOI wafer has a structure in which an oxide film 2 is placed between an upper silicon layer 1 where a semiconductor device is formed and substrate formed of a lower silicon layer 3 or the like. Oxide film 2 is referred to as a buried oxide film in some cases.
The gettering technique has an increasing importance for ensuring reliability of the semiconductor device, and thus improvement of the gettering ability is demanded.
However, the structure of the SOI wafer is not suitable for applying gettering thereto since oxide film 2 located between the upper silicon layer 1 and the gettering layer on the back prevents heavy metal from diffusing.
There is thus a problem of difficulty in executing gettering in the SOI wafer regardless of the demand for improvement of the gettering ability so as to enhance the reliability of the semiconductor device.
One object of the present invention is to provide an SOI wafer having improved gettering ability and a method of manufacturing thereof.
In order to achieve the object above, according to one aspect of the present invention, a semiconductor wafer has a gettering layer which includes a porous silicon layer.
The structure above is employed to enable a dangling bond portion of a vacancy within the porous silicon layer to function as a gettering site.
The semiconductor wafer according to the present invention preferably includes an insulating film.
The structure above is employed to implement an SOI structure having a silicon layer on the insulating film. As a result, the gettering ability can be enhanced in the wafer having the SOI structure (SOI wafer).
The semiconductor wafer according to the invention preferably has the gettering layer located above the insulating film.
By employing such a structure, gettering from the silicon layer is not interrupted by an oxide film serving as the insulating film since the gettering layer is located above the oxide film. Therefore, although it was difficult to execute gettering in the SOI wafer, the gettering ability can be enhanced in that way.
According to the invention above, preferably the porous silicon layer is in contact with the upper side of the insulating film.
By employing such a structure, the manufacturing process can be simplified since a step of forming a silicon layer between the oxide film and the porous silicon layer can be omitted.
The porous silicon layer serving as a gettering layer can be arranged away from the upper surface where the semiconductor device is located, and thus the influence of heavy metal on the semiconductor device can be reduced.
The semiconductor wafer according to the invention preferably includes a silicon layer which is in contact with the lower side of the gettering layer and in contact with the upper side of the insulating film.
By employing such a structure, the silicon layer and the oxide film can be bonded instead of the porous silicon layer and the oxide film and accordingly, bonding strength can be enhanced.
According to a preferred aspect of the invention, the semiconductor wafer has a two-layer structure constituted of a first gettering layer formed of a porous silicon layer and a second gettering layer formed of a polycrystalline silicon layer having a high concentration impurity region or a low concentration impurity region or formed of a layer containing crystal defect, and the two-layer structure is in contact with the upper side of the oxide film.
By employing such a structure as described above, gettering is carried out by a gettering layer having the two-layer structure and thus an SOI wafer having its gettering ability improved is achieved.
According to another aspect of the invention, the semiconductor wafer has the gettering layer located below the insulating film.
By employing such a structure as described above, the gettering ability can be applied to heavy metal which is diffused via the oxide film as an insulating film. Especially, if the oxide film has a relatively small thickness, the barrier function of the oxide film relative to the heavy metal is not fully exercised. In this case, the structure having the gettering layer under the oxide film is particularly advantageous. Accordingly, the gettering ability can be applied to heavy metal diffused in an SOI wafer having a thin oxide film.
According to still another aspect of the invention, the gettering layer in the semiconductor wafer has a two-layer structure constituting of a first gettering layer formed of a porous silicon layer and a second gettering layer formed of a polycrystalline silicon layer having a high concentration impurity region or a low concentration impurity region or formed of a crystal defect containing layer, and the gettering layer is in contact with the lower side of the oxide film.
By employing such a structure as described above, the gettering ability can be applied to heavy metal which is diffused in the SOI wafer having a thin oxide film. Further, the gettering ability can be enhanced since the gettering layer has the two-layer structure.
According to another preferred aspect of the invention, the gettering layer is formed of a low concentration impurity layer, and a polycrystalline silicon layer or a single crystal silicon layer which is in contact with the lower side of the low concentration impurity layer and has a thickness of about 0.01 xcexcm to 3 xcexcm. The polycrystalline silicon layer has a high concentration impurity region containing impurities of high concentration that define n-type or p-type such as phosphorus, arsenic, antimony, boron and indium or a low concentration impurity region containing impurities of low concentration such as those listed above. The single crystal silicon layer contains those impurities of high concentration. The gettering layer is in contact with the lower side of the insulating film.
By employing such a structure as described above, the polycrystalline silicon layer having the high concentration impurity region containing impurities of high concentration which define n- or p-type or the low concentration impurity region containing those impurities of low concentration, or the single crystal silicon layer containing those impurities of high concentration has a sufficiently small thickness of about 0.01 xcexcm to 3 xcexcm. The interface between layers functions as a gettering site. Therefore, particularly in this structure, the interface between those layers functions effectively as a gettering site since each layer is thin. In this way, this structure can improve the gettering ability.
The low concentration impurity region is placed between the oxide film and the polycrystalline silicon layer having the high concentration impurity region or the low concentration impurity region or the single crystal silicon layer containing those impurities of high concentration. Accordingly, the influence of impurities diffused in high-temperature process on the silicon layer constituting the semiconductor device can be prevented.
According to a further aspect of the invention, the gettering layer includes a back side porous silicon layer placed on the back side of the semiconductor wafer.
By employing this structure, a semiconductor wafer with improved gettering ability is achieved.
According to a further aspect of the invention, the gettering layer includes a back side two-layer structure placed on the back side of the semiconductor wafer that is constituted of a first gettering layer formed of the back side porous silicon layer and a second gettering layer formed of a polycrystalline silicon layer having a high concentration impurity region containing impurities of high concentration that define n- or p-type such as phosphorous, arsenic, antimony, boron and indium or having a low concentration impurity region containing those impurities of low concentration or formed of a crystal defect containing layer.
By employing such a structure, a semiconductor wafer having improved gettering ability can be obtained. In addition, the two-layer structure of the gettering layer further improves the gettering ability.
According to a further aspect of the invention, the porous silicon layer is placed in a selected region.
By employing such a structure, bonding strength can be reinforced between the porous silicon layer and the oxide film, since bonding is made not only between the oxide film and the porous silicon layer but between the oxide film and both of the silicon layer and the porous silicon layer.
According to a further aspect of the invention, the semiconductor wafer has a gettering layer including a polycrystalline silicon layer containing impurities of low concentration of 1xc3x971018 cmxe2x88x923 or less, and has an insulating film. The gettering layer is in contact with the lower side of the insulating film.
By employing the structure above, the gettering ability can be improved and enhancement is accomplished in prevention of junction leakage, reliability of the oxide film, and yield as compared with a silicon wafer according to the conventional technique. In addition, the thickness of the oxide film as well as parasitic capacity can be decreased, although it is impossible when the high concentration impurity layer is provided. In this case, no influence is exerted on the silicon area where the semiconductor device is formed.
According to the present invention, a semiconductor device is implemented on the semiconductor wafer as described above.
By employing the structure above, the semiconductor device is implemented using the SOI wafer with improved gettering ability, so that a semiconductor device of a high reliability is achieved.
According to one aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method which includes the steps of fabricating an upper wafer by forming a porous silicon layer on the upper surface of a silicon layer, turning the upper wafer upside down and overlaying it on a lower wafer formed of a silicon layer with an insulating film placed on its upper surface to bond the upper and lower wafers together, and removing an upper portion of the silicon layer which constitutes the upper wafer leaving a desired amount thereof so as to reduce the thickness of the upper wafer.
By employing those steps, an SOI wafer formed of multiple layers with improved gettering ability can be manufactured efficiently.
According to another aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method including the steps of fabricating an upper wafer by a) forming a porous silicon layer on the upper surface of a silicon layer and b) forming a first epitaxial silicon layer on the upper surface of the porous silicon layer through epitaxial growth, turning the upper wafer upside down and overlaying it on a lower wafer formed of a silicon layer with an insulating film placed on its upper surface, and removing an upper portion of the silicon layer which constitutes the upper wafer leaving a desired amount thereof so as to reduce the thickness of the upper wafer.
By employing those steps, bonding strength can be enhanced since bonding is made not between the porous silicon layer and the oxide film but between the first epitaxial silicon layer and the oxide film.
Preferably, the invention described above is characterized in that the bonding method follows a hydrogen implantation separation method.
By employing such an arrangement, the cost can be reduced compared with the method to reduce the thickness of the wafer by polishing, since the separated portion of the silicon substrate can be recycled.
According to still another aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method which includes the steps of fabricating an upper wafer by a) forming a first porous silicon layer which is in contact with the upper surface of a silicon layer, b) forming a first epitaxial silicon layer which is in contact with the upper surface of the first porous silicon layer through epitaxial growth, c) forming a second porous silicon layer which is in contact with the upper surface of the first epitaxial silicon layer, and d) forming an oxide film on the upper surface of the second porous silicon layer, turning the upper wafer upside down and overlaying it on a lower wafer formed of a silicon layer to bond the upper and lower wafers together, and removing an upper portion of the upper wafer leaving a desired amount thereof.
By employing those steps, an SOI wafer formed of multiple layers with improved gettering ability can be manufactured easily and efficiently even when the lower wafer formed only of the silicon layer is used.
According to a further aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method which includes the steps of fabricating an upper wafer by a) forming a first porous silicon layer which is in contact with the upper surface of a silicon layer, b) forming a first epitaxial silicon layer which is in contact with the upper surface of the first porous silicon layer through epitaxial growth, c) forming a second porous silicon layer which is in contact with the upper surface of the first epitaxial silicon layer, d) forming a second epitaxial silicon layer which is in contact with the upper surface of the second porous silicon layer through epitaxial growth, and e) forming an oxide film on the upper surface of the second epitaxial silicon layer, turning the upper wafer upside down and overlaying it on a lower wafer formed of a silicon layer to bond the upper and lower wafers together, and removing an upper portion of the upper wafer leaving a desired amount thereof so as to reduce the thickness of the upper wafer.
By employing those steps, an SOI wafer having at least two silicon layers formed through the epitaxial growth to improve gettering ability can be manufactured easily and efficiently.
Preferably, according to the invention, in the steps of forming the first and second porous silicon layers respectively, density of vacancies in one of the first and second porous silicon layers is made higher than that in the other, and in the removing step, one of the first and second porous silicon layers that has a higher density of vacancies is separated.
By employing such steps, separation is more likely to occur in one of the layers having a higher vacancy density than the other, so that the position where separation occurs can be controlled. As a result, an SOI wafer having improved gettering ability owing to the bonding method can be manufactured easily and efficiently.
According to a further aspect of the invention, the method of manufacturing a semiconductor wafer is characterized in that the porous silicon layer or at least one of the first and second porous silicon layer is placed in a selected region.
By employing such a step, not only the porous silicon layer but both of the porous silicon layer and the silicon layer are joined with the upper surface of the oxide film, so that an SOI wafer with improved gettering ability can be manufactured without deterioration of bonding strength.
According to a further aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method which includes the steps of fabricating a lower wafer by forming a porous silicon layer on the upper surface of a silicon layer, turning upside down an upper wafer formed of a silicon layer with an oxide film placed on its upper surface and overlaying the upper wafer on the lower wafer to bond the upper and lower wafers together, and removing an upper portion of the silicon layer which constitutes the upper wafer leaving a desired amount thereof so as to reduce the thickness of the upper wafer.
By employing those steps, an SOI wafer formed of multiple layers having improved gettering ability can be manufactured easily and efficiently.
According to a further aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method which includes the steps of fabricating an upper wafer by implementing on the upper surface of a silicon layer a two-layer structure constituted of a first gettering layer formed of a porous silicon layer and a second gettering layer formed of a polycrystalline silicon layer which has a high concentration impurity region or a low concentration impurity region or formed of a crystal defect containing layer, turning the upper wafer upside down and overlaying it on a lower wafer formed of a silicon layer having an oxide film on its upper surface to bond the upper and lower wafers together, and removing an upper portion of the upper wafer leaving a desired amount thereof.
By employing those steps above, an SOI wafer having improved gettering ability can be manufactured efficiently owing to the gettering layer of two-layer structure located above the oxide film.
Preferably, according to the invention, the bonding method follows a hydrogen implantation separation method.
By employing the arrangement above, the separated portion of the silicon substrate can be recycled and thus the cost can be decreased, as compared with the method according to which the wafer is reduced in thickness by polishing.
According to a further aspect of the invention, a method of manufacturing a semiconductor wafer employs a bonding method which so includes the steps of turning upside down an upper wafer formed of a silicon layer having an oxide film on its upper surface and overlaying the upper wafer on a lower wafer fabricated by forming a silicon layer through epitaxial growth so as to bond the upper and lower wafers together, and removing an upper portion of the silicon layer which constitutes the upper wafer leaving a desired amount thereof so as to reduce the thickness of the upper wafer.
By employing those steps, an SOI wafer having improved gettering ability can be manufactured efficiently owing to the arrangement in which the silicon layer formed through the epitaxial growth is placed below the oxide film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.